Saving the affair partner under different name is a very common practice. They usually exist as opposite sex so Fiona becomes Finlay and Mark is Maggie- same letters to begin with is not coincidence.
Cheaters cheat, they lie,lie and lie some more even when presented with cast iron evidence. They swear on lives of their children there’s nobody else. They lie.
Cheating is mindfucking business to unsuspecting (to start with) spouse. Then they start shifting the blame,project,accuse,withdraw,gaslight and whatever.
Faced with losing the family they cry, swear on their children’s lives again „it was nothing,just sex, I love you and only you”.
Except if they had truly loved you they would have never repeatedly cheated.
Mistress is for fun,sex,laughter,wining,dining,entertainment,jokes,conversation.
Wife is for labour,child rearing,housekeeping and all daily drudgery.
The bastard is happy to take from wife and children and give them FA while lavishing the mistress with time,effort,energy and money.
It is more than middle finger and ultimate betrayal.
Sometimes the tables turn though- mistress is not interested in doing all the shitty jobs that the cheating bastard assigned to wife and disappears. Love rat ends up with vacancy on both wife and mistress positions.
Have a look at Chump Lady „Loose a Cheater,Gain A Life”. Worth reading.
Good luck OP.